Capacitor with thermosealed polymeric case for implantable medical device

ABSTRACT

An electrolytic capacitor with a polymeric housing in the form of a pocket defining a chamber, with an opening along a selected edge. The opening has opposed sides that are sealed together to provide a seam. A number of conductive layers are positioned within the chamber, and a feed-through conductor element has a first end electrically connected to the layers. An intermediate portion of the feed through passes through the seam, and an external portion extends from the housing. The housing may be vacuum formed high density polyethylene, with the feed-through contained in an elastomeric sleeve having a flattened cross section to be readily received in the seam, and to accommodate thermal expansion differences between the housing and the feedthrough. The device may be manufactured by inserting a stack of layers in the pocket, and thermally welding across the opening of the pocket on a single weld line.

FIELD OF THE INVENTION

This invention relates to electronic components for implantable medicaldevices, and more particularly to charge storage components for cardiacstimulation devices.

BACKGROUND OF THE INVENTION

Defibrillators are implanted in patients susceptible to cardiacarrhythmias or fibrillation. Such devices provide cardioversion ordefibrillation by delivering a high voltage shock to the patient'sheart, typically about 500-750V. High voltage capacitors are used indefibrillators to accumulate the high voltage charge following detectionof a tachyarrhythmia. In the effort to make implantable devices as smalland thin as possible, flat aluminum electrolytic capacitors are used.

Such a flat capacitor is disclosed in U.S. Pat. No. 5,131,388 to Plesset al., which is incorporated herein by reference. Flat capacitorsinclude a plurality of aluminum layers laminarly arranged in a stack.Each layer includes an anode and a cathode, with all of the anode layersand all of the cathode layers being commonly connected to respectiveconnectors. The layers may be cut in nearly any shape, to fit within asimilarly shaped aluminum housing designed for a particular application.Normally, the cathode layers are together connected to the housing,while the anodes are together connected to a feed-through post thattightly passes through a hole in the housing, but which is electricallyinsulated from the housing. The feed-through post serves as an externalconnector for interfacing with other components.

Flat capacitors may be provided with polymeric housings that eliminatethe need for additional insulating layers to insulate conductive layersfrom the housing, reducing total size and increasing energy density(measured in Joules/cc). Such a housing is disclosed in U.S. patentapplication Ser. No. 09/130,812, filed Aug. 7, 1998, by inventor D.Carson, which is incorporated herein by reference. This device uses aninjection molded two-part plastic “dish-and lid” housing that isultrasonically welded about its periphery. Electrical feedthrough wirespass from the interior to the exterior through holes provided at theweld line. While effective, this housing requires sidewalls that arewide enough to include mating grooves and ridges for ultrasonic welding.In addition, injection molding requires more than a minimum wallthickness for the major panels to allow molten plastic material to flowthrough the mold. These thicknesses add to the total capacitor volume,decreasing the energy density from what would otherwise be ideal. Inaddition, the ultrasonic welding process may be sensitive toout-of-tolerance part dimensions, and requires significant operator careand skill, adding to manufacturing costs.

SUMMARY OF THE INVENTION

The disclosed embodiment overcomes the limitations of the prior art byproviding an electrolytic capacitor with a polymeric housing in the formof a pocket defining a chamber, with an opening along a selected edge.The opening has opposed sides that are sealed together to provide aseam. A number of conductive layers are positioned within the chamber,and a feed-through conductor element has a first end electricallyconnected to the layers. An intermediate portion of the feed throughpasses through the seam, and an external portion extends from thehousing. The housing may be vacuum-formed high density polyethylene,with the feed through contained in an elastomeric sleeve having aflattened cross section to be readily received in the seam, and toaccommodate thermal expansion differences between the housing and thefeedthrough. The device may be manufactured by inserting a stack oflayers in the pocket, and thermally welding across the opening of thepocket on a single weld line.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an implantable defibrillator according to apreferred embodiment of the invention.

FIG. 2 is an exploded view of a capacitor according to the embodiment ofFIG. 1.

FIG. 3 is an perspective view of a capacitor according to the embodimentof FIG. 1 in an intermediate stage of manufacturing.

FIG. 4 is an enlarged edge view of a capacitor according to theembodiment of FIG. 1.

FIG. 5 is an enlarged perspective view of a capacitor according to theembodiment of FIG. 1.

FIGS. 6a-8 b are perspective views of alternative capacitor housings inopen and sealed configurations.

FIG. 9 is sectional side view of a capacitor housing according to theembodiment of FIG. 1 in an intermediate stage of manufacturing.

FIGS. 10a and 10 b are sectional side views of a capacitor at differentstages of manufacturing according to an alternative embodiment of theinvention.

FIG. 11 is a sectional side view of a defibrillator including a pair ofcapacitors according to the embodiment of FIGS. 10a and 10 b.

FIG. 12 is an exploded view of a capacitor according to an alternativeembodiment of the invention.

FIG. 13 is an enlarged side view of a capacitor according to theembodiment of FIG. 12, in an intermediate stage of manufacturing.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 illustrates a defibrillator 12 for pectoral implantation, with aportion of the housing removed to show interior components. Thedefibrillator includes an outer housing 20 that includes a lead setfeed-through connector 22 for attachment of an endocardial lead set in aheader 23. The housing 20 contains a battery cell 24, electroniccircuitry 26, and a capacitor 28. The battery provides low voltageelectrical energy to a transformer in the circuitry to charge thecapacitors so that they may provide a high voltage shock when needed.The circuitry 26 connects to the lead connector so that it may sense andanalyze electrical signals from the heart, and control the delivery ofan appropriate therapy such as a high voltage shock.

FIG. 2 illustrates an exploded view of the capacitor 28, which may bedesigned as virtually any flat shape to conform to a desired housingshape. In the preferred embodiment, it is crescent-shaped to conform toa compact, ellipsoidal outer device housing. The capacitor includes aplastic housing 32 defining a chamber 34, in which resides a capacitorstack 36.

The capacitor stack 36 is formed of a number of alternating interleavedcathode sheets and anode sheets with separator sheets therebetween. Aliquid electrolyte is introduced into the stack and impregnates theseparator sheets. The anode sheets include anode tabs 40 extending inregistration with each other beyond the cathode sheets and separatorsheets at one end of the stack 36. Similarly, the cathode sheets includecathode tabs 42 extending beyond the anode sheets and registered forconnection to each other. The cathodes, like the anodes, are connectedtogether in parallel when the respective tabs are brought together in abundle. To provide electrical contact to the anodes, a flexible aluminumanode feed-through wire 44 is connected to the anode tabs 40 and extendsaway from the stack and out of the housing. A cathode feed-through wire45 similarly extends from the cathode tabs.

The housing 32 is essentially a flat pocket formed of a seamless sheet,with opposed major flat sides 46, 50, and a single opening 52 along astraight edge of the pocket. Preferably, the housing is formed by vacuumforming of a polyolefin sheet such as high density polyethylene, or anyother suitable thermoformable material. This high density materialprovides mechanical strength and electrical insulation. The wallthickness may be between 2-20 mils, although 5-10 mils is preferred.With the vacuum forming process described below, the wall thickness willtend to vary over a single part formed from a flat sheet of constantthickness. The housing 32 is sized to closely receive the capacitorstack, with the stack occupying the chamber fully, except at the opening52.

As shown in FIG. 3, the stack is installed in the housing with the feedwires 44, 45 extending from the opening 52, which has been compressedtogether to form a seam 54 running along all or most of the edge 55. Apair of compression rollers 56 compresses together the housing sides atthe edge, capturing the feed-through wires 44, 45. The rollers rotate todraw the housing into their nip until all possible space in the chamberis eliminated. Heat is applied to weld together the seam to provide aseal. Heating may be provided by an air or radiant heat source, or byheat conducted from heated rollers or other bar heater. In alternativeembodiments, ultrasonic welding or adhesive bonding, or solvent weldingmay be employed.

FIG. 4 shows the edge 55 of the capacitor housing, with the feed-throughwire 45 captured in the seam 54. The wire has an elastomeric sleeve 56surrounding an aluminum conductor 60. The sleeve has a lenticular crosssection having an acute vertex along opposed edges. These vertices allowthe housing material to smoothly conform to the sleeve, without anyvoids where the seam line meets the sleeve. The elastomeric sleeve ispreferably formed of EPDM or similar olefenic elastomer havingcompatible properties with the housing material to form an adequatebond. Thus, the EPDM sleeve thermally bonds to the housing material. TheEPDM is further advantageous in this application in that it is athermoset material which will not melt or change shape or thicknessduring bonding. To provide a positive seal between the wire 60 and thesleeve 56, the wire may have a larger diameter than the sleeve bore,maintaining compressive contact.

The sleeve material not only provides a positive bond with the heatedhousing material, but its flexible properties accommodate any thermalexpansion differences between the plastic housing and the aluminum wire.Where such expansion differences are not of concern, the feed-throughmay be an all aluminum conductor having a flat shape, such as a foilstrip, or a drawn wire having the lenticular profile shown. A furtheradvantage of the illustrated wire shape is that the broad surfaces areless prone to penetrate or rupture through the thin housing wall duringcompressive heating, as might a small circular wire.

FIG. 4 further shows a vent element 62 welded into the seam 54. This isa strip of porous material that extends into the chamber, and which hasan edge exposed to the environment, allowing gas generated or trapped inthe chamber to slowly escape, while containing fluid within the chamber.In the preferred embodiment, the vent is a strip of PTFE felt that issufficiently hydrophobic that it does not wick the typically hydrophilicelectrolyte, and which has an adequately high melting point that it isunaffected by temperatures used to seal the housing. Alternative ventsinclude sintered PTFE or ultra-high molecular weight polyethylene(UHMWPE). Such a porous vent is preferred over diffusion membranes orplugs such as might be formed of PDMS silicone, because during vacuumcycles used for evacuation of gas, as well as gas generated duringelectrical testing, diffusion barriers do not relieve pressure quicklyenough, and the housing would expand undesirably. Even if the vent stripmaterial does not form a bond with the housing material during welding,the texture of the strip admits adequate housing material to form amechanically engaged connection adequate to prevent fluid leakage.

FIG. 4 also shows that at one end of the seam, an open portion 64 of theseal is left unsealed. This is a temporary aperture that will later besealed, but which provides a ready exit for excess trapped air duringsealing, and an inlet for electrolyte injection following initialsealing. While it may be possible to pre-saturate the stack withelectrolyte prior to insertion and sealing, it is believed that this mayimpair the sealing process, and may generate unwanted vaporization fromthe heat of sealing. Accordingly, in the preferred embodiment, thehousing is mostly sealed after stack insertion, then electrolyte isadded (preferably under a vacuum), and the remaining open portion issealed, so that little or no air occupies the chamber.

As shown in FIG. 5, the open portion 64 of the seam is away from the endof a sealing region 66 that extends across the remainder of the edge,including beyond the opposite end of the seam. The sleeve 56 of the wireneed only cover an intermediate portion passing through the sealingregion, as the vent element need only extend just beyond the sealingregion to the interior and exterior of the housing.

FIG. 6a shows an alternative housing 70 having a simple open end withstraight walls at the open edge 55. When sealed as shown in FIG. 6b, theend of the seam extends beyond the end of the rest of the housing, asthe end wall of the housing at the edge is folded outward. Where thisprotrusion is unwanted, an alternative housing 72 shown in FIG. 7a maybe provided with score lines 74 in a pattern that causes the end panelto fold inward, as shown in FIG. 7b. This gable-end technique ensuresthat there are no protrusions beyond the housing ends, and requires asealing process capable of accommodating the thicker end portion of theseams. A further alternative housing 76 is shown in FIGS. 8a and 8 b.The housing opening is essentially pursed at the ends of the opening, sothat the opening is shorter than the rest of the housing. When thisopening is flattened and sealed, the lengthening that naturally occursremains at or less than the length of the housing, avoiding protrudingseam portions.

FIG. 9 shows the vacuum forming process used to produce the housing. Avacuum table 80 has vent holes 82 beneath each of an array of positiveforms 84 that correspond to the interior shape of the housing. A heatedsheet 86 of the housing material is placed above the forms, and issealed to the edges of the vacuum table (in a manner not shown) toprevent air from being admitted between the sheet and the table. Avacuum is drawn from beneath the sheet through vents 82, and the sheetis conformed to the forms. After the sheet cools adequately, it isremoved, and the housings are individuated by cutting along cutting line90. By using male forms, the material stretching is least at the upperor peripheral edge, while material thins more significantly over themajor faces of each housing, more so near the open ends. This providesstructural strength at the edge walls, but reduced thickness over thelarge areas, providing a greater effect on volume reduction. These thinmajor walls are readily made by vacuum forming, while an injectionmolding process would have the greatest difficulty with a smallthickness over a large area. Where it is necessary to avoid excessivelythin or thick regions, the sheet may be pre-molded to provide increasedor reduced thickness to compensate as needed.

FIGS. 10a and 10 b show an alternative housing configuration 92 havingan opening 94 with an upper wall portion 96 extending well beyond themajor surface 100 from which it upstands. An opposed wall portion 102 isco planar with the opposed major surface 104. During sealing, the firstwall portion is folded across to meet the unmoved portion 102, so thatthe seam 106 extends in the plane of the major surface 104. As shown inFIG. 11, two such capacitors may be connected back to back and installedin the capacitor housing, so that the seams are adjacent. This allowssimple electrical connection of the capacitors by conductor 110, andprovides usable volumes 112 adjacent the seam and contiguous with therest of the device volume for occupation by other circuitry.Alternatively, the seams may be placed against opposite walls of thehousing, and the space between efficiently occupied by circuitry toreduce overall device volume.

FIG. 12 shows an alternative housing configuration 114 having apocket-type housing 116, but with a separate lid 120. The lid is sizedto fit the opening 122 along the straight edge of the housing, and to besealed about the periphery of the opening. This alternative does notrequire folding of the housing edge, which may be straight as it extendsbeyond the inserted capacitor stack 124. The lid 120 has a planar bodythat fills the opening, and an upstanding peripheral flange 126. Asshown in FIG. 13, the lid is installed so that the body is nearlytouching the edge of the stack 124, with the flanges 126 terminating atthe edges of the housing walls. The device is inserted into a die 130having a pocket sized to closely receive the capacitor, and a tool 132is inserted into the space defined by the flange of the lid. With thelid firmly positioned, any of the sealing methods noted above may beapplied, including radiant, convective, or conductive heat, as well asultrasonic energy, adhesive, or solvent welding. The lead wires may bewelded into the seams as discussed above. In an alternative embodiment,instead of the vacuum formed lid shown, a molded plastic lid may beprovided. Such a lid may have more precise detailed features, such asapertures that accept feed-through wires.

Although the above invention is described in terms of a preferredembodiment, the invention is not intended to be so limited.

What is claimed is:
 1. An electrolytic capacitor comprising: a polymerichousing comprising a pocket defining a chamber and having an openingalong a selected edge; the opening comprising opposed sides sealedtogether to provide a scam; a plurality of conductive layers positionedwithin the chamber; and a feed-through conductor element having a firstend electrically connected to the conductive layers, an intermediateportion passing through the seam, and an external portion extending fromthe housing.
 2. The capacitor of claim 1 wherein at least theintermediate portion of the feedthrough includes a surrounding sleeve,and wherein the sleeve material is different from the conductivefeedthrough material.
 3. The capacitor of claim 2 wherein the sleeve hasan elongated cross section at the intermediate portion.
 4. The capacitorof claim 2 wherein the sleeve is an elastomeric material.
 5. Thecapacitor of claim 1 wherein the housing is formed of high densitypolyethylene.
 6. The capacitor of claim 1 wherein the seam is a singleline.
 7. The capacitor of claim 1 including a vent element capturedwithin the seam.
 8. The capacitor of claim 1 including a second separatefeed-through element.